Which of these numbers is prime? ${33,\ 38,\ 51,\ 55,\ 73}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 33 are 1, 3, 11, and 33. The factors of 38 are 1, 2, 19, and 38. The factors of 51 are 1, 3, 17, and 51. The factors of 55 are 1, 5, 11, and 55. The factors of 73 are 1 and 73. Thus, 73 is a prime number.